An apparatus for the production of blown synthetic-resin foils and films generally comprises a blowing head provided down-stream of a plasticizing or extrusion press, a calibrating basket, a flattening device, a drawing or drafting device and a coiling or winding station.
The blowing head generally receives a stream of plastified or plasticized synthetic-resin material and extrudes the latter through an annular nozzle as a tube which is inflated by a stream of air introduced into the tube to expand it to the desired degree, thereby thinning the wall of the tube to foil or film thickness.
The calibrating basket is designed to control the degree to which the tube is inflated and hence controls the wall thickness of the "bubble" or "balloon", while the flattening device brings opposite walls of the tube together to flatten the latter into a continuous band and, simultaneously, seal the upper end of the blowing space into which the air is introduced. The drawing device insures a continuous tension on the flat band or web which is coiled into rolls at the coiling or winding station.
The blowing head generally includes a blower for internal cooling and inflating air and for external cooling air, the internal and external air streams cooling the thermoplastic material so as to stabilize the foil at the desired thickness.
A control device is generally provided for the internal cooling air, the external cooling air and, therefore, the air supply and discharge from the "bubble".
The calibrating basket also usually includes a height-adjusting device which positions the basket relative to the frost line, i.e. the level at which the wall of the thermoplastic tube is congealed to the desired degree. The calibrating basket is provided with a diameter-control device which regulated the maximum diameter of the blow tube, i.e. the balloon formed by expansion of the extruded tube, so that the wall thickness will remain constant and at the desired level and, finally, a measuring device responsive to the foil thickness is provided between the drawing device and the coiling or winding station to control the thickness.
The degree of filling of the balloon is regulated by a measuring device responsive to the degree of filling and acting via control members on the controller for the blowing head while the foil-thickness controller, which responses to the thickness measuring device, operates on the drawing speed via appropriate control members acting upon the drawing rollers.
The blowing head referred to above includes not only the nozzle through which the initial synthetic-resin tube is extruded, but also the associated elements such as the blowing device, etc.
In the conventional systems of this type, the circuit regulating the degree of filling of the blown balloon of synthetic-resin material is a proportional control system. The balloon is sensed and the displacement of the sensor is applied as an input signal to a displacement/pressure transducer. The output signal of this displacement/pressure transducer is applied to a control cylinder as the controlling element of the system. The servomechanism which responds to this control signal varies the quantity of internally supplied air.
This arrangement has certain disdvantages. For example, should disturbances or error signals be generated in this control system, they cannot be readily reduced to zero by the control in the manner described. Disturbances may arise from various sources. For example, they may be the result of changes in the temperature of the synthetic-resin mass, or changes in the room temperature because of night-to-day differences. They may be a function of the through-put of the synthetic-resin material, the quantity of external cooling air which determines the frost line, the foil thickness and the like. In general, one attempts to the greatest possible extent to maintain constant all of the parameters of the blowing process and, especially, to maintain constant the filling of the foil balloon via the calibration basket to maintain the diameter of the balloon and hence the flattened width of the foil constant.
A change in the breadth of the foil generally results in an adjustment of the calibrating basket which can be provided with a scale from which the breadth of the flattened foil can be determined. The control, however, usually is effected by a manual measurement and a manual adjustment of the diameter of the calibrating basket to achieve the desired tolerances in the flattened foil width.
Nevertheless, various degrees of shrinkage may occur or other disturbances can affect the flattened width.
The measuring devices for the foil thickness have been provided in various forms. The foil generally moves past a head prior to coiling or winding. As a result, the measurement of the thickness takes place at a location relatively remote from the basket so that significant dead time or response time between changes in foil thickness and a response of the control system can exist. Such response lags may be between two to three minutes and can even amount to as much as 20 minutes. As a consequence, defects in the product must always be taken into account.
As a rule, in conventional apparatus of the type described previously, the control circuit for the degree of filling of the blown balloon and the foil-thickness control circuit function independently from one another although they have mutual effects upon one another which may result in one of the systems disrupting the other.
As a consequence, it is not possible to control the apparatus within narrow tolerances completely to maintain a predetermined foil thickness and a predetermined foil breadth in the systems known heretofore. For this reason, operating personnel must constantly monitor the system, generally on an empirical basis, in order to maintain the desired degree of control.